1. Humans are not generally capable of sufficiently accurate long-term low-incidence risk assessment. Meaning, you irrationally value potentially getting to work 10 seconds faster over a 50% increased chance you run over a child crossing the street.
2. Humans are subject to too many irrational psychological factors; stuff like:
• False sense of security due to sitting in a box isolated from the outside world, that's advertised to keep them "safe" in case of a collision.
• Herd mentality, e.g. "everyone's going over the limit, so I will too". Bonus points for rationalizing this behavior "because it's safer to go at the speed of traffic!".
• Delusional rationalizations like "if the limit is 50 then going 10 over must be fine too, due to <reasons>!". Bonus points for applying the "5/10/15/20 over" rule for every possible speed limit — basic maths and physics say hello!
3. The speed humans will travel at on a given road depends primarily on what speed that road seems designed for. People will drive faster on straight, wide roads and slower on winding, narrow ones, regardless of the speed limit. Changing speed limits has little effect compared to changing the physical infrastructure. Show me a picture of a road and I'll tell you how fast people will drive on it.
As such, it makes no sense to first make some sort of a road and only then figure out the limits by observing real traffic. Figure out the appropriate limit first, then design the road with it in mind.
But that's true (look up the Solomon curve), and it's exactly why the 85th percentile would be better.
> Delusional rationalizations like "if the limit is 50 then going 10 over must be fine too, due to <reasons>!". Bonus points for applying the "5/10/15/20 over" rule for every possible speed limit — basic maths and physics say hello!
You have cause and effect backwards. People think it's safe to go over the speed limit precisely because most speed limits are too low.
> Changing speed limits has little effect compared to changing the physical infrastructure. Show me a picture of a road and I'll tell you how fast people will drive on it.
Right. So even if going slower is safer, just making the speed limit lower won't accomplish that.
That's all the more reason to raise speed limits on the major roads. Speed limits being more reasonable there makes it more likely that drivers would abide by them even on those smaller residential streets.
